Reinforced rotative abrasive structures



Aprll 24, 1962 A. E. RAYMOND 3,030,743

REINFORCED ROTATIVE ABRASIVE STRUCTURES Original Filed June 14, 1956 @Imxke abras/i/e sfrucfure IN V EN TOR. ,gzafrffiflrwa/vp W MQ G fan United States Patent Ofiice 3,030,743 REINFORCED ROTATIVE ABRASIVE STRUCTURES Albert E. Raymond, St. Paul, Minn, assignor to Minnesota Mining and Manufacturing Company, St. Paul, Minn, a corporation of Delaware Continuation of application Ser. No. 591,357, June 14, 1956. This application Aug. 6, 1958, Ser. No. 753,862

Claims. (Cl. 51-207) application Serial No. 400,565, filed December 28, 1953,.

now abandoned.

Abrasive wheels, discs, and the like, are required to operate at high rotative speeds and accordingly are subject to failure occasioned by the high centrifugal force developed. Failure may occur through cracking of the entire wheel, or by throwing out of edge segments of the wheel, or in other ways. These abrasive articles are also subject to flexural failure caused by flexing of the disc or wheel under pressures asserted on the sides thereof during abrading operations. Here again the article may fail by cracking of the wall section on flexing, followed by throwing out of segments, or even by complete disintegration, of the wheel. The prevention of centrifugal and fiexnral failure of rotative abrasive articles is there fore a primaryobject of the present invention.

One way which has been suggested for the strengthening of abrasive discs and the like is by the incorporation in the structure of glass fibers or filaments in the form of glass cloth or as short segments of fibers or filaments. Glass filaments are high in strength and low in stretch and may be obtained in any desired diameter. However, glass filaments are weakened excessively by any slightest scratch or abrasion of their surfaces such as may be occasioned by rubbing against overlapping strands, and thereafter are nonresistant to flexing and bending. Short fibers tend to produce bulky layers and to pull away from the surrounding resin and therefore do not provide the desired high strength per unit of volume. The application of continuous glass filaments in a manner which causes no contact with adjacent filaments and which leaves the filaments lying in straight non-wavy lines, so as to permit the development of the full strength of the glass in the finished article, is therefore another major object of the present invention.

These and other objects and advantages are attained in the novel structures which, together with exemplary methods of making the same, will hereinafter be described, and in which continuous straight glass filaments are held in closely adjacent parallel position Within individual layers, the filaments being bonded to and separated from each other by minimum proportions of strong tough abradable bonding resins to provide centrifugally and flexurally reinforced layers in rotative abrasive structures.

In the drawing, FIGURE 1 is a perspective view of a rotative abrasive structure made in accordance with this invention; FIGURES 2-8 are partial cross-sectional views of a number of typical rotative abrasive structures; and FIGURE 9 is a cross-sectional view of a filament-reinforced component of such structure togethere with its temporary carrier or supporting web.

The article of FIGURE 9 consists of a smooth kraft paper carrier 92 treated on one surface with a low adhesion coating 93, e.g., of polyethylene, and coated with a tough resinous binder 91 containing as reinforcing ele ments a plurality of glass yarns as each composed of a large number of very small individual glass filaments, the filaments and the yarns being laid parallel to one another and without substantial bending or twisting. The yarns are completely impregnated, and the filaments completely individually surrounded, by the resinous unifying binder 91. The binder is partially cured to a nontacky flexible state, and the combination of binder and yarns is easily removable from the treated surface of the paper carrier as a flexible self-supporting sheet material.

The manner in which the filament reinforced self-sup- Y porting resinous sheet material employed in the article of FIGURE 9 is prepared and resinous compositionswhich can be employed therein will now be shown with the aid of the following illustrative examples.

Example I A formulation for the binder 91 consisting of the following components in the indicated proportions by weight is first prepared:

i Rubbery butadiene-acrylonitrile copolymer 100,

Zinc oxide Phenol-aldehyde thermosetting resin (Super Beckacite 1003 50 Vinsol ester gum 50' Salicylic acid 15' Dibutyl phthalate 7 .9

The components are homogeneously blended together in approximately 400 parts of methyl ethyl ketone to provide a liquid composition suitable for coating on the previously prepared carrier web. Sufficient of the liquid material is applied to the carrier to provide a dry weight of about 10 grains per 24 sq. in. and is dried at room or slightly ele-. vated temperatures. The surface of the dried binder layer is then re-activated to a tacky state by moistening with methyl ethyl ketone, and a layer of glass yarn, consisting,- for example, of 70 ends per inch of width of a yarn containing 204 filaments per end, preferably an untwisted or very slightly twisted yarn, is laid on the sticky resin surface. At 70 ends per inch, the yarns cover substan-. tially the entire surface of the resin layer. Fewer yarns may be used where maximum strength is not required. Where desired, a filament density of somewhat greater than 70 ends per inch may be employed. Single filaments may be applied, but the use of yarns or bundles of filaments is more convenient and is preferred.

A further coating of the binder solution, amounting to about 12 grains per 24 sq. in. dry weight, is next applied, the final proportion of glass to resin then being about 22 grains of glass and about 22 grains of binder per 24 sq. in. of sheet material. The resin solution impregnates the yarn and flows around the individual filaments. Still further amounts of binder solution may be applied if desired; this is desirable particularly where more than about 70 ends per inch of glass yarn are used in order to insure proper resin encasement of the individual filaments. Somewhat smaller amounts are also effective. The solvent is removed by evaporation and the heatcuring resinous composition is partially cured by heating. In the reinforcing sheet thus constructed, each filament is separately supported in the resin Without kinking or bending and substantially without contacting any adjacent filaments; and each filament is continuous throughout the entire length of the sheet material. The binder forms an elastic and strongly adherent protective layer Patented Apr. 24, 1.962

3 around each filament. Other heat-advancing tough adherent rubbery binders, such as combinations of other rubbery polymers and heat-advancing tackifier resins compatible therewith, may be substituted for the exemplary formula here provided.

A further coating consisting of heat-curing resinous grit-binder, e.g., a phenolic resin grit-binder, and abrasive grit material in the desired proportion and in any desired total quantity may then be applied to the exposed surface of the partially cured reinforced resinous coating of the structure of FIGURE 9, and the resinous binder then partially cured to provide a sheet material which may be used in building up more complicated rotative abrasive structures. The resinous binder may be in the form of a solution of a phenolic resin and the abrasive grit material may be spread over the surface of such coating; or the abrasive grit material may be pre-mixed with the binder solution and the two applied together; or a dry mixture of abrasive grit material and fusible grit-binder particles may be spread over the reinforced sheet and heated to provide the desired intermediate product; or the abrasive grit material may first be placed on the sheet and then bonded thereto by subsequent addition of the phenolic resin binder in any desired manner. The abrasive gn't may be applied in a bulk layer, or may be applied as clusters or individually spaced granules. Any of the well-known grit materials, e.g., silicon carbide, aluminous oxide, garnet, etc. may be used, in any desired grit size.

The specific grit-binder employed will depend on the particular abrading application for which the product is designed, and may be any one of a Wide variety of known materials of which the phenolic resin grit-binders, as used in conventional abrasive discs and abrasive wheels, represent a preferred class. These binders are hard and tough, and are considerably less readily stretchable than the rubbery resinous binder surrounding the glass filaments of the sheet described in the present example. Grit-binders may be used which are too weak to provide adequate centrifugal strength by themselves but which when reinforced with the filamentous sheet material provide fully effective rotative abrasive structurcs. The binders are strongly adherent both to the abrasive grit and to the reinforcing sheet material.

Instead of reinforced sheet material containing a rubbery resinous binder, a reinforced sheet material having a considerably greater concentration of glass yarns and employing a resinous binder which in cured or thermoset condition is quite hard and tough and essentially inelastic, i.e., non-rubbery, is advantageously employed. Such a structure will now be described.

Example 11 p A stable heat-curing resin composition is first prepared by uniformly mixing at about 200 F. 15 parts by weight of finely powdered diallyl melamine in 100 parts of an epoxide resin having a melting point of about 40 C., an epoxide equivalency within a range of 300-375 and hydroxyl equivalency of about 105 (grams of resin per gram-equivalent of reactive OH radical). The mixture is held at about 200 F. during subsequent operations, at which temperature the pot life is found to be at least about 2 hours. A flat compact web containing approximately 200 ends of parallel glass yarns per inch over its 28 inch width and formed by drawing the yarns from eight warp beams mounted in parallel, each containing 700 ends of lightly twisted 204-filament glass yarns in a beam width of 54 inches, through condensing combs is then impregnated with the liquid epoxide resin composition by drawing the web through the liquid resin and between squeeze rolls. The product is thereafter rapidly chilled. The squeeze rolls are adjusted such that the resulting sheet material contains about 60 percent of glass by weight.

The sheet material exhibits a non-tacky surface and can be wound upon itself in roll form for storage and subsequently unwound therefrom for use. Preferably, however, where the sheet material is to be stored for extended periods of time it is preferable to store it in contact with a releasable liner such, for example, as the liner employed in connection with the article described in Example I. The sheet material can be readily split lengthwise, the resin being essentially inelastic, although it is capable of substantial bending, folding and general handling without undue separation into narrow strips of tape or filament bundles. The resin remains stable indefinitely when stored at room temperature, or preferably, somewhat below room temperature. The resin is readily cured to its hard inelastic state upon subjection to heat and pressure, at least 25 pounds per square inch pressure and 330 F. temperature for about 20 minutes being suitable.

A coating consisting of heat-advancing resinous gritbinder and abrasive grit material may then be applied to the surface of the reinforced resinous sheet material and the resinous binder then partially cured through the application of heat to provide a sheet material which may be used in building up more complicated rotative abrasive structures, in accordance with the teachings hereof.

Other resin compositions which heat-cure to a tough, hard abradable and essentially inelastic state, such as alkyd type resin compositions, phenol-aldehyde resins, etc., may be substituted in lieu of the epoxide resin composition in the reinforced resinous sheet material of the present example. However, where a hard inelastic resin is to be employed it is preferable that somewhat greater concentrations of filament yarns be employed than is necessary where rubbery resinous compositions, such as that described in the previous example, are used. For example, where less than about -150 ends of filament yarn are employed with the inelastic resins described, the resulting resinous sheet material is less easily handled without longitudinal splitting than sheet materials containing greater concentrations of filament yarns. On the other hand, where the concentrations are more than about 300 ends per inch the liquid resinous composition does not as effectively penetrate the glass-filament web so as to encase the individual filaments. The 200 ends per inch in the sheet material of the present example is a preferred concentration.

As previously noted, the abrasive coated reinforced sheet material of FIGURE 9, with or without additional uncoated reinforced sheet material, can be advantageously employed in many rotative abrasive structures. Several of these will now be described with the aid of the accompanying drawing.

The structure indicated in FIGURE 2 consists of three separate layers 20, 21 and 22 of the filament reinforced resinous sheet material of FIGURE 9 and a further surface layer 23 of abrasive grit and resinous binder. The three filament-containing layers are laid up in such a way that the filaments of each are at an angle of 60 with the filaments of each of the others, i.e., with substantially equal angles between filament directions. The entire assembly is consolidated and the resin cured under heat and pressure into a dense structure substantially free of voids and having very high tensile strength in all directions within the plane of the sheet. In the form of a rotative abrasive structure this sheet material is highly resistive to centrifugal failure as well as flexural failure.

In making the sheet material of FIGURE 2 the three separate layers of filament reinforced resinout binder, made as described in connection with FIGURE 9, are removed from the carrier (if a carrier has been used) and laid up as indicated. A mixture of abrasive grit material and heat-advancing resinous binder in the desired proportion and amount is spread over the surface of the upper ply and the entire structure is then subjected to heat and pressure, for example in a suitable mold between heated platens in a hydraulic press.

In an alternative procedure a single layer of filament reinforced resinous sheet material illustrated in FIGURE 9 is first coated with a layer of abrasive grit and binder, the binder is partially cured, and the resulting sheet is then removed from the carrier web and combined with two additional layers of uncoated filament reinforced resinous sheet material, the whole being pressed and heated as before. The single layer carrying the abrasive coating may be conveniently produced in a continuous process and wound into storage rolls for subsequent combining with the other layers. Or, if desired, several layers of uncoated filament reinforced resinous sheet material can first be combined and unified by heat and/ or pressure, the abrasive grit and binder being thereafter applied.

Instead of only partially curing the one or more layers of filament reinforced sheet material prior to or after application of the coating of abrasive grit and binder, such that the resins are finally cured later during formation of the integrated structures, these may be fully cured prior to formation of the integrated articles. In this event, however, it will generally be necessary to employ in the appropriate fashion a separate unifying adhesive to facilitate unification of the more complicated integrated structures.

The structure indicated in FIGURE 3 is similar to that of FIGURE 2 in having three filamentary layers 38, 31 and 32 and a surface abrasive layer 33, but in addition has another surface abrasive layer 34 on the side opposite the first abrasive layer. The structure may be prepared by either of the alternative methods suggested in connection with FIGURE 2.

In FIGURE 4, two filament reinforced layers 40 and 41 previously coated with abrasive layers 42 and 43, respectively, and with the longitudinal direction of the parallel filaments of the two layers at right angles to each other, are separated by 'a relatively thick resin bonded abrasive layer 44. FIGURE 5 is similar but omits the outer abrasive layers 42 and 43 of FIGURE 4 and is designed particularly for applications involving only edgecutting. Rotative structures as here illustrated are effective at relatively small diameters or thick sections, but structures such as that of FIGURE 5 in particular are sometimes found to warp or buckle when made in large diameter and thin section with only one or two thicknesses of filaments at each of filament layers 5t and 51; and multiple-thickness filament-reinforcing layers are generally preferred.

The filament reinforced resinous film of FIGURE 9 may also be used in conjunction with other reinforcing components such as the interior cloth layer 60 of FIG- URE 6. This cloth layer 61 which may be a glass cloth, is preferably first impregnated with a suitable heat-setting resinous composition and the filament reinforced resinous webs 61 and 62, carrying the resin bonded abrasive coatings 63 and 64., respectively, are bonded to the surfaces of the cloth member under heat and pressure.

In these and other rotative abrasive structures it will, of course, be apparent that where single layers of the filament reinforced resinous web are indicated in many of the figures, multiple layers may equally well be employed and in most cases are desirable in order to provide additional centrifugal and flexural strength. In all of these structures, containing at least two layers of the reinforcing web in either adjacent or non-adjacent positions, the several Webs are to be laid with their filament directions substantially at equal angle to each other in order to provide maximum uniformity of centrifugal strength. Furthermore in thin, large-diameter structures such as that of FIGURE 5 in which reinforcing filaments are present at or near both flat surfaces of the structure, each of the reinforcing layers is composed of at least two and preferably three or more layers of filament-reinforced resin with their filament directions at equal angles, as

hereinbefore noted, in order to avoid warping and to provide maximum flexural strength at all portions of the disc and in all directions of fieX.

FIGURE 7 illustrates a further modification in which four filament reinforced webs 70, 71, 72 and 73 with their associated bonded abrasive layers 74, 75, 76 and 77, respectively, are combined in a single abrasive structure together with a flexible resinous layer 78, this last named layer being free of abrasive grit material and imparting additional flexibility and toughness to the structure. A compatible mixture of heat-advancing phenol-aldehyde resin and polyvinyl butyral has been found effective for such use; such mixtures are also useful in forming the filamentous web itself.

Still a further modification is shown in FIGURE 8. The structure here illustrated consists of six filament reinforced layers each with an associated resin bonded abrasive layer and additionally containing three flexible resinous layers 8d, 81 and 82. The flexible resinous layers which may be of the formula just indicated in connection with layer 78 of FIGURE 7 improve the flexibility of the structure and provide additional bonding to the back surface of the filament reinforced layers where the amount of resin contained therein is otherwise inadequate. It will also be observed in the structure shown in FIG- URE 8 that two opposing surfaces of previously applied resin bonded abrasive such as are indicated at 83 and 84 are caused to bond together to form a single integral layer between the two filament-containing layers 85 and 86.

The rotative abrasive structure of FIGURE 1 may be produced from the structures indicated in FIGURES 2-8 either by forming and molding as an individual unit as already described, or by cutting or punching the discshaped article from a continuous web of uncured sheet stock having the cross-sectional structure indicated and then curing in a mold, or by pre-curing the structure in flat sheet form and then cutting to shape.

The glass filaments provide an extremely high strengthto-volume ratio, yet are readily disintegrated and removed under the abrading action at the edge of the wheel or disc when in use. Glass filaments are therefore preferred. However in some instances adequate centrifugal and flexural strength may be attained with other filaments which, due to their internal positioning in the disc and to the type of application for which the disc is designed, need not be rapidly worn away during the useful life of the article. In such cases metal wires and cellulosic or other organic polymeric filaments may replace the glass filaments. The resinous compositions employed are also abradable. That is, they are removed with the abrasive waste as the article is consumed through use without clogging or gumming the abrasive surfaces.

Generally, a temperature and pressure suificient to cure the phenolic-resin grit-binder will also adequately cure the binder resin of the reinforced sheet material. Hence, cure conditions in the main are governed by the characteristics of the former. Typical curing conditions which have been found satisfactory in some instances, e.g., in forming a double coated abrasive disc as illustrated in FIGURE 6 and employing a heat-curing phenolic resin grit-binder, comprise heating for two hours at 212 F. followed by 5 hours at 250 F. under a pressure of 1500 lbs. on a disc having a 6 inch diameter. The surfaces of the heated platens are initially treated with a silicone parting agent so that the phenolic resin does not adhere thereto and so that the completed disc may be easily removed.

In another instance a composite sheet prepared as indicated in FIGURE 3 in the form of a square sheet 5 inches on each side and made with a phenolic grit-bond was heated for 5 /2 hours at 215 F. and for 5 hours at 275 F. under a pressure of 1500 lbs. on a 6 inch diameter ram, and a disc having an outside diameter of 3 inches and with a one-half inch diameter central hole was then cut from the resulting slab. The curing time and temperature indicated provides a cure suflicient to permit removal and cutting of the slab but does not necessarily develop the highest possible strength. The resulting slabs or discs are therefore further cured where necessary.

In some instances a green pre-form may be produced under much lower pressures and without heating and the resulting intermediate product then separately cured in an oven. A structure having the cross-section of FIG- URE 7 was made by such a procedure, the composite being preliminarily pressed under a pressure of 250 lbs. applied on a 6 inch ram and at room temperature and the pressed green slab then being cured in an oven for 1 /2 hours at 175 F. followed by 45 minutes at 325 F.

Abrasive discs, cut-off wheels, grinding wheels and other rotative abrasive structures made in accordance with the foregoing principles and disclosure are found to have extremely high resistance to centrifugal and fiexural failure and to be highly effective for the purposes for which they are designed. Since the glass filaments are permitted to lie in straight lines rather than in a series of zigzag bends as in woven fabrics or the like they impart their full tensile strength to the structure immediately on the application of any stress and without first permitting stretching of the sheet. The individual filaments are protected and separated by the surrounding layer of resinous binder and hence they do not become abraded or scratched, and their full strength is maintained. While some few of the filaments may be broken or in short lengths, the filaments may be considered substantially continuous and unbroken and hence there is a minimum possibility that separation of filament and surrounding resinous binder may occur. The extreme thinness of the individual filaments and of the bundles or yarns as they are incorporated in the sheet material makes possible the attainment of maximum strength in all directions throughout the plane of the sheet while maintaining minimum thickness, as contrasted to structures employing woven fabrics in which each individual filament lies in what corresponds to a plurality of layers and which must therefore inherently contain voids or points of weakness throughout the filamentous structure.

Others have concerned themselves with the employment of glass filaments or fibers in the manufacture of reinforced resinous articles and sheet materials. For example Palm Patent No. 2,322,771, granted June 29, 1943, Francis Patent No. 2,602,776, granted July 8, 1952, Tallman Patent No. 2,552,1M granted May 8, 1951, and British Patent No. 619,674 disclose structures utilizing glass filaments or fibers as reinforcing in one manner or another. However, none of these patents is concerned with the reinforcement of abrasive structures. Moreover, insofar as I am aware, no one prior to the present invention has provided or disclosed reinforced abrasive structures, or backings therefor, containing a plurality of layers of resin-bonded nonwoven lineally-aligned bundles of lightly twisted densely packed continuous fine glass filaments individually coated with a fusible, rapidly thermosetting, strongly adherent resinous composition stable under storage conditions which is potentially heatcurable to a hard, tough, dense resin firmly encasing said filaments. The present invention furthermore avoids the difficulties and deficiencies of previously known structures and provides novel and useful abrasive coated discs, abrasive cut-off wheels, abrasive grinding wheels, and other rotative abrasive structures as illustratively, but not limitatively, defined and described hereinabove.

What I claim is as follows:

1. A fiat unitary reinforced rotative abrasive structure of the nature of an abrasive wheel or disc, having high centrifugal and fiexural strength, having at least one layer of bonded abrasive grit and at least two separate layers of straight parallel unwoven reinforcing filaments individually encased in a flexible adhesive binder, the several layers of filaments being incorporated in said structure as pre-formed self-supporting filament-reinforced rubbery'resinous sheet material, the longitudinal direction of the parallel filaments of the several layers of filaments being at substantially equal angles with each other.

2. The fiat unitary reinforced rotative abrasive structure of claim 1 in which two layers of straight parallel reinforcing filaments are united to opposite flat surfaces of a layer of bonded abrasive grit.

3. The flat unitary reinforced rotative abrasive structure of claim 1 in which a plurality of layers of straight parallel reinforcing filaments are disposed at substantially equal angular relationship to each other in a unitary reinforcing layer bonded to one flat surface of a layer of bonded abrasive grit.

4. The flat unitary reinforced rotative abrasive structure of claim 3 in which the unitary reinforcing layer is bonded at each side to a fiat surface of a layer of bonded abrasive grit.

5. The method of making a flat unitary reinforced rotative abrasive structure of the nature of an abrasive wheel or disc and having high centrifugal and fiexural strength, comprising building up a green pre-form of separate adjoining layers of pre-formed self-supporting reinforcing web and of mixed abrasive grit and thermosetting binder, and unifying and curing said pre-form under heat and pressure, said web being comprised of straight parallel unwoven reinforcing filaments individually encased in a flexible adhesive binder, there being at least two layers of said web, and with the longitudinal directions of the parallel filaments of said layers being at substantially equal angles to each other.

6. The method of making a pre-formed abrasive structure suitable for use in the formation of flat unitary rotative abrasive structures of the nature of abrasive wheels or discs, which have high centrifugal and fiexural strength, formed of separate adjoining layers of said preformed structure, the method comprising laying up into a superimposed stack at least two layers of a self-supporting reinforcing web comprised of straight parallel unwoven reinforcing filaments individually encased in a flexible adhesive binder with the longitudinal directions of the parallel filaments of said layers being at substantially equal angles to each other, unifying said stack, applying abrasive grains and thermosetting binder therefor to at least one surface of said stack and at least partially curing said thermosetting binder to unify the structure.

7. A flat unitary reinforced rotative abrasive structure of the nature of an abrasive wheel or disc, having high centrifugal and fiexural strength, having at least one layer of bonded abrasive grit and at least two separate layers of straight parallel unwoven reinforcing filaments individually encased in a strong abradable unifying resinous adhesive binder, the several layers of filaments being incorporated in said structure as pre-formed selfsupporting filament-reinforced resinous sheet material, the longitudinal direction of the parallel filaments of the several layers of filaments being at substantially equal angles with each other.

8. A flat unitary reinforced rotative abrasive structure of the nature of an abrasive Wheel or disc, having high centrifugal and fiexural strength, having at least on layer of bonded abrasive grit and at least two separate layers of straight parallel unwoven reinforcing filaments individually encased in a tough, hard, strong abradable resinous adhesive binder, the several layers of filaments being incorporated in said structure as preformed selfsupporting filament-reinforced resinous sheet material, the longitudinal direction of the parallel filaments of the several layers of filaments being at substantially equal angles with each other.

9. A reinforced abrasive sheet structure suitable for use in the manufacture of a flat unitary reinforced rotative abrasive structure of the nature of an abrasive wheel or disc comprising abrasive grains associated with a backing of a preformed thin flexible self-sustaining sheet material consisting of a plurality of layers of resinbonded nonwoven lineally-aligned bundles of lightly twisted continuous fine glass filaments in each layer of which the glass filaments are closely and densely packed together, each filament being completely coated, surrounded and individually encased, and bonded to each other by a fusible, rapidly thermosetting, strongly adherent resinous composition stable under storage conditions, said filaments being substantially uniformly distributed throughout each said layer of resin-bonded sheet material and comprising at least about 60 percent of the Weight thereof with each layer having substantially a single thickness of said bundles of glass filaments, and said resinous composition being potentially heat-curable to a hard, tough, dense resin firmly encasing said filaments and making up 40 to 20 percent of said layer of resin-bonded sheet material, each said layer of sheet material being capable of being laid up in a sheet comprising several layers with the longitudinal direction of the parallel filaments of the several layers being at substantially equal angles With each other, and the resin converted under heat and pressure to a cured state, thereby causing the several layers to bond firmly together into a unitary structure which resists delamination under the flexing encountered during use of an abrasive disc.

10. A flat unitary reinforced rotative abrasive structure of the nature of an abrasive Wheel or disc, having high centrifugal and fiexural strength, having at least one layer of bonded abrasive grit and backing of at least two preformed layers of resin-bonded nonwoven lineallyaligned bundles of lightly twisted continuous fine glass filaments in each layer of which the glass filaments are closely and densely packed together, each filament being completely coated, surrounded and individually encased, and bonded to each other by a fusible, rapidly thermosetting, strongly adherent resinous composition stable under storage conditions, said filaments being substantially uniformly distributed throughout each said layer of resinbonded sheet material and comprising a least about 60 percent of the Weight thereof with each layer having substantially a single thickness of said bundles of glass filaments, and said resinous composition being potentially heat-curable to a hard, tough, dense resin firmly encasing said filaments, the longitudinal direction of the parallel filaments of the several layers being at substantially equal angles with each other and the resin converted to a hard cured state.

References Cited in the file of this patent UNITED STATES PATENTS 2,232,389 Jurkat Feb. 18, 1941 2,284,716 Benner et al June 2, 1942 2,284,739 Hurst June 2, 1942 2,337,445 Buell Dec. 21, 1943 2,375,263 Upper May 8, 1945 2,643,494 Erickson June 30, 1953 

